1. Field of the Invention
The present invention relates to a rotatable image-bearing belt position detecting device for image forming apparatus.
2. Description of the Related Arts
A conventional full-color copying machine is provided a transfer belt for transferring the toner image formed on the surface of the photosensitive member.
Three primary color toner images on for example, yellow, cyan and magenta, are sequentially transferred onto the aforesaid transfer belt so as to be overlaid thereon and form a color image.
The rotation of the transfer belt and the movement of the scanner while scanning the original document must be synchronized so as to precisely overlay the three primary color toner images on the transfer belt. Therefore, the travel position (rotation position) of the transfer belt must be detected with a high degree of precision.
Conventionally, in order to detect the transfer belt position, a pulse generator disk having a slit and a sensor to generate pulse signals in accordance with the rotation of the aforesaid disk are provided on the motor shaft or the roller shaft for drivingly moving the transfer belt The aforesaid pulse signals are counted to detect the belt position.
In the previously described conventional position detection device, however, a position detection disk is mounted on the shaft of the roller or the like, such that dislocation may be produced between said disk and the transfer belt due to slippage and flexion between the roller and the belt or external oscillation and, thus, position detection cannot be accomplished to a high degree of precision.
Therefore, the movement of the transfer belt and the movement of the scanner cannot be precisely synchronized due to the aforesaid slippage. The lack of precise synchronization results in dislocation of the toner image on the transfer belt and, ultimately, image color blurring, thereby reducing image quality.
In view of the previously described disadvantages, the provision of a detection mark on the transfer belt itself has been considered as a method for preventing the reduction in detection precision caused by slippage between the roller and the transfer belt.
Precise detection of the belt position is difficult, however, when a mark provided on the belt itself is detected by a sensor due to the gradual, not rapid, rise in the sensor detection signal.
In the previously described belt position detection device wherein the belt position is detected by counting pulse signals, the position of the belt in the direction of rotation of the transfer belt can be detected, however slippage of the transfer belt perpendicularly to the direction of rotation cannot be detected.
When the transfer belt is dislocated perpendicularly to the direction of rotation the toner images formed on the transfer belt also become dislocated as they overlay one another. The aforesaid dislocation causes image color blurring and poor image quality.
In conventional copying machines, copying is executed even when lateral oscillation of the transfer belt occurs because said lateral oscillation of the belt is not detected, resulting in disadvantages such as the formation of lot quality images, unnecessary toner consumption and wasted time.